What happens when there is no electricity?

When there is a power failure, the alarm will revert to a back-up battery so that the premises are still protected. The battery life will vary depending on the size of the system but there is usually up to 3 hours of backup. Once the electricity is restored, the backup battery charges back from the electricity supply. Moreover, some systems can alert you on a mobile phone device when there is a power failure and when this has been restored.